Reliability of Isokinetic Knee Strength Measurements Using Genneralizability Theory. The Journal of Kinesiology, 2017, 19(4): 29-35. [PURPOSE] Isokinetic strength test is the measurement of force-velocity or torque angle. Isokinetic strength of the knee extensor and flexor muscle is often assessed using an isokinetic dynamometor such as Cybex system. Question remains concerning the reliability of the isokinetic muscle strength measurements. The purpose of this study was to establish the minimum number of trial required to assess reliable estimates of isokinetic strength measurement using Cybex system(Cybex 770). [METHODS] A total of 70 adults (mean age 19.38 ± 3.32 years; 35 male and female each) completed the isokinetic strength tests using Cybex system. Concentric muscle strengths were measured at 60 degrees per second over 4 repetitions. Following data screening, participant’s data were finally analyzed by muscle strength measurements (knee extensor and flexor muscles) and gender using the Genealizability theory. A single-facet crossed design [i.e., participant(P) × trial(T)] was used. G-study was performed to quantify the percentage of variance associated with each facet and interaction in the model. Follow-up D-study was performed to determine the optimal combinations of number of repetition trial to achieve a desirable reliability coefficient (G≥.80). [RESULTS] The knee extensor muscle of male and female were 158.48 ± 33.89Nm and 96.78 ± 23.60Nm respectively. The knee flexor muscle of male and female were 83.52 ± 31.92Nm and 45.97 ± 11.94Nm, respectively. For the knee extensor muscle, the results from G-study show that P accounted for 71% and 90% variance for male and female. T had no effect on the total variance. The number of trials to achieve G≥.80 for male and female were 2 and 1, respectively. For the knee muscle G-study results show that P accounted for 94% and 77% variance for male and female. T had little effect on the total variance (< 1%). The number of trials to achieve G≥.80 for male and female were 1 and 2, respectively. [CONCLUSIONS] The number of examination for reliable values is at least 2 times in isokinetic knee strength test.